The loss of its cyclin causes a Cdk to do what?

The correct response to the question is based on the role of cyclins in regulating cyclin-dependent kinases (Cdks). Cyclins are crucial regulatory proteins that bind to Cdks to activate them. When a Cdk is associated with a cyclin, it undergoes a conformational change that enables it to phosphorylate target proteins, which is vital for cell cycle progression.

When the cyclin is lost, the Cdk is unable to maintain its active conformation. This loss means that the Cdk can no longer effectively phosphorylate its target proteins, leading to its inactivation. The functionality of Cdks is intrinsically linked to the presence of their cyclin partners; hence, without cyclins, Cdks revert to an inactive state.

This mechanism is essential in cell cycle regulation, as it ensures that the cell only progresses through the cycle phases when appropriate conditions are met, thereby maintaining proper cellular function and division. The inactivation of Cdks upon the loss of cyclin is a key part of the regulatory process that controls cell division and response to cellular signals.